In Killer Comet, you are in a hostile region of space, and beset by space monsters who are trying to devour you and your space-pod. The monsters launch bombs, and contact with these or the monsters themselves will result in the loss of one of your five space-pods. The monsters themselves move fairly randomly, drifting slowly towards the bottom of the screen. Occasionally, the glimmering orb of a killer comet will appear and will purposefully follow you as you move to avoid it. Monsters, bombs and comets are all vulnerable to a short-range laser beam which you carry on-board, and may fire in one of three different directions. If things get too difficult, you can also trust to luck and enter hyper-space, which will teleport your pod to a random location in space.
Monsters appear in waves and after killing 25, a new type will appear. The player score is the total number of monsters vanquished.